In ResGroupDropFinish() uninitialized memory address can be accessed due
to some reasons:

1. the group pointer is not initialized on segments;
2. the hash table node pointed by group is recycled in removeGroup();

This invalid access can cause crash issue on segments.

Also move some global vars to resgroup.c,
They were put in resgroup-ops-linux.c, which was only compiled and
linked on linux, so on other OS like macos the vars can not be found.